Conventional electrolytes, which are used for electrochemical devices such as chargers, capacitors and sensors, have been prepared as solutions or pastes for better electric conductivity. However, these devices are likely to be damaged due to leakage of liquid and it is difficult to reduce the size and volume of electrolytes because a separator is required with liquid electrolytes. In order to solve the above-mentioned problems, the solid electrolytes prepared by using inorganic crystalline substances, inorganic glasses and organic polymers have been developed. The organic polymers are normally superior in processing characteristics and molding characteristics and the resulting solid electrolytes have flexibility and bending processing characteristics. Design freedom of the device for which the solid polymer electrolyte is applied becomes large. However, the organic polymer substance is inferior to other materials in ion conductivity at present.
In order to solve such problems, numerous efforts have been made to develop various solid polymer electrolytes, pure solid polymer electrolytes, gel-type polymer electrolytes, hybrid polymer electrolytes and the like.
Polymer matrixes of gel-type solid polymer electrolyte and hybrid polymer electrolyte are impregnated with excessive amount of electrolytic solution to obtain good ionic conductivity. However, some electrolytic solution impregnated into the polymer matrix can be leaked and reduce the characteristics as batteries.
On the other hand, since an ionic conductivity of pure solid polymer electrolyte can be obtained by local segmental motion, pure solid polymer electrolyte prepared by using a polyether and a plasticizer salt has no leakage of electrolytic solution but has a reduced ionic conductivity. Grafted solid polymer electrolytes having polyoxyethylene side chains with low molecular weight for rapid chain migration have also been introduced to solve such problems.
For example, a method for manufacturing a solid polymer electrolyte comprising an alkali metal salt and a polymeric adduct prepared by reacting hydroxy-functional acrylic copolymer and polyether monoisocyanate is disclosed in U.S. Pat. No. 5,337,184. The resulting solid polymer electrolyte has improved flexibility but the acrylic copolymer has no contribution to improve ionic conductivity.
Recently, a polyether copolymer having oligooxyethylene side chains as a solid polymer electrolyte has been disclosed in Japanese Laid-Open Publication Nos. 63-154736 and 63-241026, European Patent No. 434011 and U.S. Pat. No. 5,837,157. Although these solid polymer electrolytes show good ionic conductivity at low temperature, they show poor mechanical characteristics and the processes for preparing them are complicated.
There have been also two conventional methods for preparing poly(N-substituted urethane). The first method of solution or melt polymerization is disclosed in German Patent Nos. 1,720,693; 1,720,706; and 1,720761. For example, N-methyl polyurethane having high molecular weight is prepared by polycondensation of α,α′-(4,4-di-N-methylaminodiphenyl)-p-diisopropyl benzene and bisphenol A bis(chlorocarbonic ester) in two-phase reaction medium comprising aqueous sodium hydride solution and methylene chloride/chlorobenzene mixture.
The other method for preparing poly(N-substituted urethane) has been developed by the formation of polyurethane sodium salt through removing active hydrogen by using sodium hydride or sodium and then nucleophilic substitution reaction with an alkyl halide. For example, polyurethane comprising ethylene glycol and methylenebis(phenyl isocyanate) (MDI) dissolved in dimethylformamide (DMF) is dropped into sodium hydride dissolved in DMF to give polyurethane sodium salt. After one hour, the sodium salt is reacted with methyl iodide dissolved in DMF under inert gas atmosphere at 0° C. for two hours to produce the corresponding N-methyl polyurethane (See H. C. Beachell and J. C. Peterson Buck, J. Polym. Sci., Polym. Chem., Ed. 7, “dilute solution studies of nitrogen-substituted polyurethanes”, 1873-1879, 1969).
However, it has been reported that it is impossible to obtain carbamates corresponding to primary amines with the above-mentioned first method. Although the second method may complement the problem associated with the first method, the sodium hydride having strong basicity used in the second method invites the dissociation of urethanes as a side-reaction and thus, results in a reduced molecular weight of the final product, reduced physical properties and low yield of poly(N-substituted urethane). For example, a conversion rate to poly(N-methyl urethane) in the reaction of ethylene glycol and MDI is about 50% and when a substituent having higher molecular weight than methyl group is replaced for methyl, it is even less than 15% (See H. C. Beachell and J. C. Peterson Buck, J. Polym. Sci., Polym. Chem., Ed. 7, “dilute solution studies of nitrogen-substituted polyurethanes”, 1873-1879, 1969). The poly(N-substituted urethane) has lower mechanical properties than polyurethane and thus it requires an introduction of crosslinking structure.
Further, polyethyleneoxide-grafted polyurethane copolymer may be used for N-substitution reaction in the first method. However, the applications to solid polymer electrolytes obtained by using a polyether copolymer matrix having oligooxyethylene side chains are restricted due to the above-mentioned side reaction and low conversion rate.
Therefore, there are increasing demands for preparing a polyether poly(N-substituted urethane) copolymer which provides an appropriate ionic conductivity and mechanical characteristics and is useful polymer matrix of solid polymer electrolytes by easily controlling the length of side chains, concentrations, compositions, structures and crosslinked degrees.